Methods of Discovery and Measurements for Small Cells in OFDM/OFDMA Systems

ABSTRACT

A method of small cell discovery and RSRP/RSRQ measurements in OFDM/OFDMA systems is proposed. A discovery reference signal (DRS) with low transmission frequency is introduced to support small cell detection within a short time, multiple small cell discovery, and accurate measurement of multiple small cells. The DRS consists of one or multiple reference signal types with the functionalities including timing and frequency synchronization, cell detection, RSRP/RSSI/RSRQ measurements, and interference mitigation. RE muting is configured for the DRS to reduce interference level from data to DRS for discovery and RSRP/RSRQ measurements for small cells.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 from U.S.Provisional Application No. 61/883,336, entitled “Methods of Discoveryand Measurements for Small Cells in OFDM/OFDMA Systems,” filed on Sep.27, 2013; U.S. Provisional Application No. 61/968,491, entitled “Methodsof Discovery and Measurements for Small Cells in OFDM/OFDMA Systems,”filed on Mar. 21, 2014, the subject matter of which is incorporatedherein by reference.

TECHNICAL FIELD

The disclosed embodiments relate generally to OFDM/OFDMA systems, and,more particularly, to discovery and measurements for small cells inOFDM/OFDMA systems.

BACKGROUND

In 3GPP Long-Term Evolution (LTE) networks, an evolved universalterrestrial radio access network (E-UTRAN) includes a plurality of basestations, e.g., evolved Node-Bs (eNBs) communicating with a plurality ofmobile stations referred as user equipment (UEs). Orthogonal FrequencyDivision Multiple Access (OFDMA) has been selected for LTE downlink (DL)radio access scheme due to its robustness to multipath fading, higherspectral efficiency, and bandwidth scalability. Multiple access in thedownlink is achieved by assigning different sub-bands (i.e., groups ofsubcarriers, denoted as resource blocks (RBs)) of the system bandwidthto individual users based on their existing channel condition.

In 3GPP Release-11 LTE system, time-domain muting scheme together withinterference cancellation receiver techniques are utilized forinter-cell interference coordination/cancellation to enable the cellrange extension of picocells for better mobile data offloading frommacrocell in the heterogeneous networks (HetNet), where there aredeployments of both macrocells and picocells sharing the same frequencyband. In addition, coordinated multi-point (CoMP) operation is alsoenabled to provide more system throughput gain with more tightlycooperation among base stations in HetNet. For further improvement ofsystem throughput, wide deployment of small cells in the mobile networksis viewed as a promising technology and feature in 3GPP Release 12 LTEsystem. Small cells generally include picocells, hotspot, femtocells,and microcells in licensed band and WiFi AP in unlicensed band.

Unlike macrocell with a coverage radius ranging from one to severalkilometers, small cells are low-power radio access nodes that operate ineither licensed or unlicensed spectrum with a coverage radius rangingfrom tens to hundreds of meters. With emerging needs for more systemthroughput due to the popularity of smart phones, many mobile networkoperators are eagerly looking for methods to enhance the utilizationefficiency of available radio spectrum by either spectrum efficiencyimprovement in licensed band or mobile data offloading in unlicensedband. As a technology providing promising gain in radio spectrumutilization efficiency, deployment of small cells receives broadattention from mobile network operators in recent years and 3GPP isplanning to enable small cell deployment in the next release of LTEsystem.

In 3GPP Release-12 LTE system, the techniques to enable the deploymentof small cells in licensed band will be the focus in RAN working groups.Due to possible acquisition of 3.5 GHz frequency bands, it enables thepossibility of non-cochannel deployments for small cells to reliefinterference issues between macrocells and small cells. As one ofconsidered scenarios, the signaling overhead of mobility management andthe time radio access interruption due to handover can be improved byassigning the frequency band for the deployment of macrocells as amobility layer and the other frequency band for the deployment of smallcells as a capacity layer. In addition to non-cochannel deployment,further enhancements on the inter-cell interferencecoordination/cancellation techniques are also considered and underevaluation for cochannel deployment.

With large number of small cell deployments, new techniques are neededto resolve possible issues in both protocol and physical layers, such asmobility management, inter-cell interference handling, on/off small celloperation, etc. In addition, how to enable smooth migration of legacyUEs with limited impact is also one of important issues. The techniquefor efficient small cell discovery is one of the techniques remain underevaluation and development.

To support small cell on/off operation for the mitigation of inter-cellinterference due to cell-specific reference signals (CRS) and loadshifting among small cells, discovery of multiple small cells withinlimited time is needed and discovery reference signal (DRS) was proposedin 3GPP to enable it. One of candidate solutions for discovery signaldesign is to reuse existing reference signal designs. However, theysuffer from high inter-cell interference level or large reference signaloverhead. In addition, timing/frequency synchronization offset betweensmall cells also affects the performance of cell detection andmeasurement using some existing reference signal designs. Therefore,enhancements are needed to resolve aforementioned problems when existingreference signal design is reused for small cell discovery.

SUMMARY

A method of small cell discovery and RSRP/RSRQ measurements inOFDM/OFDMA systems is proposed. A discovery reference signal (DRS) withlow transmission frequency is introduced to support small cell detectionwithin a short time, multiple small cell discovery, and accuratemeasurement of multiple small cells. The DRS consists of one or multiplereference signal types with the functionalities including timing andfrequency synchronization, cell detection, RSRP/RSSI/RSRQ measurements,and interference mitigation. RE muting is configured for the DRS toreduce interference level from data to DRS for discovery and RSRP/RSRQmeasurements for small cells.

In a first novel aspect, a base station allocates a set of resourceelements (REs) over multiple time-domain OFDM symbols in a set of timeslots or subframes for transmitting a corresponding set of DRS from aplurality of small cells. The base station transmits configurationinformation to a plurality of user equipments (UEs). The configurationinformation comprises information on DRS duration, DRS periodicity, andRE muting patterns of the set of REs for the plurality of small cells.In one embodiment, the DRS comprises at least one of a cell-specificreference signal (CRS), a channel state information reference signal(CSI-RS), a positioning reference signal (PRS), and a synchronizationsignal (PSS/SSS).

In a second novel aspect, a base station transmits a set of discoveryreference signal (DRS) on a corresponding set of resource elements (REs)over multiple time-domain OFDM symbols in a set of time slots orsubframes. The base station obtains DRS configuration informationincluding DRS duration, DRS periodicity, and RE muting patterns of theset of REs. The base station applies RE muting for data transmission ona first subset of the set of REs that are not used for DRS transmission.The base station applies full power data transmission on a second subsetof REs. The RE muting patterns may be determined based on cell loading,and no RE muting is applied in control channels or when collide withother legacy reference signals. The RE muting patterns may be receivedfrom another base station, or determined based on PCI. In oneembodiment, the first subset of REs belongs to a first subset of themultiple time-domain OFDM symbols, and the second subset of REs belongsto a second subset of the multiple time-domain OFDM symbols.

In a third novel aspect, a user equipment (UE) receives a set ofdiscovery reference signals (DRS) on a corresponding set of resourceelements (REs) over multiple time-domain OFDM symbols in a set of timeslots. The UE obtains DRS configuration information. The UE performssynchronization and cell detection using a subset of DRS REs based onthe configuration information. The UE performs measurements usinganother subset of DRS REs based on the configuration information. In oneembodiment, the UE performs a first measurement on a first subset oftime-domain OFDM symbols to obtain a first metric, and the UE performs asecond measurement on a second subset of time domain OFDM symbols toobtain a second metric. The first metric is a Reference Signal ReceivedPower (RSRP), and RE muting is applied on a subset of REs that are notused for DRS transmission in the first subset of time-domain OFDMsymbols. The second metric is a Received Signal Strength Indicator(RSSI), and RE muting is not applied on REs in the second subset oftime-domain OFDM symbols. In one embodiment, a DRS duration lasts one ormore subframes, and the DRS is transmitted with a periodicity that issubstantially longer than one radio frame. The UE performssynchronization, cell detection, and measurements within one single DRSduration.

Other embodiments and advantages are described in the detaileddescription below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1A illustrates a heterogeneous network deployed with both microcell and small cell in accordance with one novel aspect.

FIG. 1B illustrates two examples of subframe structure with normal andextended CP in 3GPP LTE systems based on OFDMA downlink.

FIG. 2A illustrates small cell discovery using Discovery ReferenceSignal (DRS).

FIG. 2B illustrates different DRS design considerations with RE mutingfor enhanced cell discovery and measurements.

FIG. 3A illustrates a user equipment (UE) performing synchronization,cell detection, and measurement for small cells.

FIG. 3B illustrates simplified block diagrams of a base station and auser equipment in accordance with embodiments of the present invention.

FIG. 4 illustrates a first embodiment of DRS design for small celldiscovery and measurement.

FIG. 5 illustrates a second embodiment of DRS design for small celldiscovery and measurement.

FIG. 6 illustrates a third embodiment of DRS design for small celldiscovery and measurement.

FIG. 7 illustrates a fourth embodiment of DRS design for small celldiscovery and measurement.

FIG. 8 illustrates a fifth embodiment of DRS design for small celldiscovery and measurement.

FIG. 9 illustrates a sixth embodiment of DRS design for small celldiscovery and measurement.

FIG. 10 illustrates a seventh embodiment of DRS design for small celldiscovery and measurement.

FIG. 11 illustrates an eighth embodiment of DRS design for small celldiscovery and measurement.

FIG. 12 illustrates a ninth embodiment of DRS design for small celldiscovery and measurement.

FIG. 13 illustrates a tenth embodiment of DRS design for small celldiscovery and measurement.

FIG. 14 illustrates an eleventh embodiment of DRS design for small celldiscovery and measurement.

FIG. 15 is a flow chart of a method of resource allocation andconfiguration for DRS from eNB perspective in accordance with one novelaspect.

FIG. 16 is a flow chart of a method of small cell discover andmeasurement from eNB perspective in accordance with one novel aspect.

FIG. 17 is a flow chart of a method of small cell discover andmeasurement from UE perspective in accordance with one novel aspect.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1A illustrates a heterogeneous network 100 deployed with both microcell and small cell in accordance with one novel aspect. HetNet 100 is a3GPP LTE mobile communication network comprising a user equipment UE101, a macro base station MeNB 102, and a plurality of small basestation SeNB 103, SeNB 104, and SeNB 105. In 3GPP LTE system based onOFDMA downlink, the radio resource is partitioned into radio frames,each of which consists of 10 subframes. Each subframe has a time lengthof 1 ms and is comprised of two slots and each slot has seven OFDMAsymbols in the case of normal Cyclic Prefix (CP) and six OFDMA symbolsin case of extended CP. Each OFDMA symbol further consists of a numberof OFDMA subcarriers depending on the system bandwidth. The basic unitof the resource grid is called Resource Element (RE), which spans anOFDMA subcarrier over one OFDMA symbol. A physical resource block (PRB)consists of 12 subcarriers in frequency domain and 1 slot in timedomain, which constitutes 84 REs in normal CP and 72 REs in extended CP.Two PRBs locating in the same frequency location spans in differentslots within a subframe is called a PRB pair. Error! Reference sourcenot found.B illustrates two examples of subframe structure for bothnormal and extended CP based on OFDMA downlink.

When an UE is turned on in a cell or handovers to a cell, it performsdownlink synchronization and system information acquisition beforeconducting random access process to get RRC-layer connected. Downlinksynchronization is performed by an UE with primary and secondarysynchronization signals (PSS and SSS) to synchronize the carrierfrequency and align OFDM symbol boundary between the base station of acell and an UE. Further frequency and timing fine-tune or tracking iscarried out continuously with cell-specific reference signal (CRS) by anUE. CRS is a kind of common pilots that are always transmitted in wholechannel bandwidth in every subframe no matter whether there is datatransmission. When there is data transmission, CRS is not pre-coded witha MIMO precoder even if MIMO precoding is applied so CRS can also beutilized for the coherent data demodulation when there is precodinginformation provided to an UE. In addition to CRS, UE-specific referencesignals (DMRS), which are a kind of dedicated pilots, are also specifiedin Release 8/9/10/11 LTE systems. Compared to CRS, DMRS is onlytransmitted in the radio resources where there is data transmission andit is pre-coded with the same MIMO precoder together with the data tonesfor a specific UE if MIMO precoding is applied and it is mainly utilizedfor coherent data demodulation.

After an UE gets downlink synchronized, system information acquisitionis the next step to obtain necessary information for random access andconnection/service settings. For the best trade-off between transmissionoverhead and connection delay, system information is divided intoseveral blocks in LTE system, each of which has different periodicities.Master information block (MIB) is one of system information blocks andcontains information of downlink cell bandwidth, system frame number(SFN), physical HARQ indicator channel (PHICH) configuration and thenumber of transmit antenna ports. MIB is carried in physical broadcastchannel (PBCH), which is transmitted every radio frame with a fixedperiodicity of four radio frames. After obtaining MIB, UE is able toobtain system information block SIB1 and other SIBs for further systemsetting. SIB1 and other SIBs are carried in physical downlink sharedchannel (PDSCH), which is scheduled by downlink physical control channel(PDSCH). SIB1 is transmitted every second radio frame with a fixedperiodicity of eight radio frames while other SIBs has variableperiodicity configured in SIB1.

In small cell deployments, both hotspot and hotzone are possiblescenarios. In LTE Release 12, hotzone scenario is the focus and bothsparse and dense small cell deployments within a hotzone are consideredfor different traffic density requirements. For ubiquitous coverage,both indoor and outdoor small cell deployments are considered as well.Since user distribution may vary with time and places within a hotzone(e.g. more users in an office building during the day time and lessusers during the night time; higher user density in the stores on salein a department), small cell on-off operation and load balancing amongsmall cells may be required for network power efficiency, interferencecontrol and higher user throughput.

To support the discovery of a small cell which is in non-active state(i.e. a state with no or limited signal transmission) and the smallcells which are not the small cell with strongest received signalstrength for an user (e.g. small cells with 2^(nd), 3^(rd) or 4^(th)strongest received signal strength), a special reference signal, calleddiscovery reference signal (DRS) may be needed. One of the main reasonsto introduce non-active state to a small cell is to reduce theinter-cell interference between small cells due to the transmission ofCRS when there is no data transmission. From the perspectives ofinter-cell interference, it is preferred to introduce DRS with the muchless transmission frequency than CRS. For the power efficiency of a userequipment (UE), it is also preferred to introduce DRS with which UE candiscover a small cell in a short time. For the load balancing betweensmall cells, it is preferred for the DRS to support the discovery andaccurate measurements of multiple small cells. The measurements mayinclude reference signal received power (RSRP), reference signalreceived quality (RSRQ), channel state information (CSI) and other newmeasurement metrics. Referring back to FIG. 1A, UE 101 first establishesa radio resource control (RRC) connection with MeNB 102 and receivesconfiguration information related to DRS transmission for small cells.After obtaining the DRS configuration information, UE 101 is then ableto perform small cell discovery and measurements based on the DRStransmitted from the SeNBs.

The overall objectives of DRS can be summarized as follows. First, DRShas low transmission frequency. Second, DRS supports the detection ofsmall cell within a short time. Third, DRS supports the discovery ofmultiple small cells. Fourth, DRS supports accurate measurements ofmultiple small cells. A DRS can consist of one or multiple referencesignal types, and each reference signal type served for one or multiplepurposes. To minimize the impact on UE implementation, reusing ormodifying existing reference signals is preferred for the design of DRSin small cell discovery. FIG. 1B illustrates one example of DRS 110 in asubframe with normal CP. DRS 110 comprises existing reference signalsincluding PSS/SSS pilot pattern for synchronization, antenna-port-0 CRSpilot pattern for RSRQ measurement, and antenna-port-15 CSI-RS pilotpattern with RE muting for RSRP measurement.

FIG. 2A illustrates small cell discovery using Discovery ReferenceSignal (DRS). Compared to reference signals for cell detection, RRM/CSImeasurements and demodulation, the transmission periodicity of DRS canbe much longer. As illustrated in FIG. 2A, DRS is transmitted with aperiodicity of 100 ms in a synchronous way among different small cellsSC#0, SC#1, and SC#2. With this design, the reference signal overhead,the interference introduced due to reference signals and the consumed UEpower for both cell detection and UE measurements can be largelyreduced.

However, since CRS is reused in DRS, the existing CRS scheme suffersfrom large inter-cell interference due to its frequency-reuse-oneutilization. Though frequency-reuse-one utilization minimizes theoverhead, it requires long-time averaging to achieve the requirements ofmeasurement or detection accuracy. Without any alteration to CRS, itcannot fit the design considerations of DRS. It is observed that RSRPmeasurement performance improves when the average cell loadingdecreases. From FIG. 2A, it can be seen that CRS suffers from two typesof interference—1) CRS interference from neighboring cells as depictedby arrow 201, and 2) Data interference from neighboring cells asdepicted by arrow 202. With the technique of CRS interferencecancellation, CRS interference from neighboring cells can be minimizedand the only remaining interference is data interference fromneighboring cells. Therefore, when the average cell loading in smallcell layer decreases, data interference from neighboring small cellsdecreases as well and the RSRP measurement performance improves.Nevertheless, it is uncertain that average cell loading in small celllayer is always small to guarantee minimal data interference foraccurate RSRP measurement.

In accordance with one novel aspect, RE muting technique is utilized toachieve the same effect artificially and the best trade-off between theoverhead of RE muting and the data interference level suffered by CRScan be achieved by the network configuration based on the average cellloading in small cell layer. For example, when the average cell loadingis high, more REs can be muted to guarantee the required performance forsmall cell discovery and measurements. When the average cell loading islow, fewer REs can be muted. The same technique can be applied whenreusing positioning reference signal (PRS), and channel-stateinformation reference signal (CSI-RS) as well. In current system, REsthat are potential locations for PRS transmission within a time slot areall muted except those for actual PRS transmission in a cell. It is alsopossible to configure one or several subsets of REs that are potentiallocations for non-zero-power CSI-RS (NZP CSI-RS) transmission aszero-power CSI-RS (ZP CSI-RS) to provide orthogonal radio resources forNZP CSI-RS transmission among different cells.

Furthermore, received signal strength indicator (RSSI) is a kind ofmeasurement to reflect the average cell loading on a carrier frequencyand the measurement of RSSI needs to be carried out on the radioresources where all cells transmit signals. If RE muting is applied tomute the data interference from the neighboring cells to CRS, there isno way for UE to measure RSSI on CRS. The same situation happens for PRSand CSI-RS. Therefore, it is proposed to leave part of REs that can beused for DRS transmission unmuted and UE can measure RSSI on thoseunmuted REs.

FIG. 2B illustrates various detailed DRS design considerations. First,RE muting is applied for enhanced cell detection and signal strengthmeasurement, as depicted by box 210. To improve the performance of celldetection and signal strength (e.g. RSRP) measurement for UEs, REs thatare potential locations for DRS transmission within a time slot and arenot used for actual DRS transmission in a cell can be partially or allmuted in the cell to control the interference level introduced to UEsserved by other cells for DRS reception. The time slots for DRStransmission can be one or multiple subframes in 3GPP LTE system. The REmuting pattern can be configured to an UE for data reception by eitherhigher-layer signaling from an eNB or by a predefined rule. Thepredefined rule can be based on the physical cell identification (PCI).

In one embodiment, RE muting patterns in different cells can be assignedthrough coordination so that DRS in different cells suffer zerointerference due to the orthogonally created by RE muting patterns. Ifthe RE muting pattern is configured to an UE by higher-layer signaling,the coordination is done through inter-eNB communication for dynamic orsemi-static adaptation. If the RE muting pattern is configured to an UEbased on PCI, the coordination is done through PCI assignments. Inanother embodiment, RE muting patterns in different cells can also beassigned without coordination. Each cell can pick its own RE mutingpattern randomly based on a targeted interference level to other cells.After averaging, random RE muting patterns in different cells can thusprovide an interference environment with the targeted interferencelevel. For example, if current average cell loading is 50% and itrequires the interference level introduced by 25% average cell loadingto meet the performance requirements, 50% of REs that are potentiallocations for DRS transmission and are not used for DRS transmission canbe randomly selected for muting in each cell. The RE muting pattern forDRS in a cell can vary based on the index of the time slot fortime-domain hopping to randomize the introduced interference accordingto a predefined rule or higher-layer signaling.

Second, for minimized performance impact on legacy UEs, RE muting forreduced interference to DRS should be applied to data channel only, asdepicted by box 220. No RE muting is applied in control channels, suchas 0, PCFICH, PHICH and PDCCH in LTE system. This is because theadditional RE muting may reduce the coding rate and degrade the decodingperformance of these control channels for legacy UEs. For EPDCCH, REmuting can be applied and the introduced decoding performance impact iscontrolled by either applying MIMO beamforming transmission or utilizingPDCCH as a fallback mode. Even with restricted RE muting for controlchannels, the average interference level on DRS still can be reduced forthe performance enhancement on cell detection and measurement. WheneverRE muting collides with legacy reference signals, no RE muting can beapplied. To avoid the collision with legacy reference signals, DRS canbe transmitted in subframes where there are no or limited legacyreference signals. For example, in 3GPP LTE system, DRS can betransmitted in MBSFN subframes to avoid the collision with CRS andPSS/SSS.

Third, RE muting is restricted to support RSRQ measurement, as depictedby box 230. For the measurement of total received power or receivedsignal strength indicator (RSSI), RE muting is not preferred because REmuting artificially reduces the interference level due to datatransmission. To support the measurement of total received power orreceived signal strength indicator (RSSI) in the time slots or OFDMsymbols where DRS exists, part of REs which are potential locations forDRS transmission are further reserved and no RE muting is applied onthem. Therefore, REs that are potential locations for DRS transmissionare partitioned into two sets of REs. In the first set of REs, REsexcept those for DRS transmission can be partially or all muted for themeasurement of reference signal received power or RSRP. The second setof REs are reserved for the measurement of total received power orreceived signal strength indicator (RSSI) and no RE muting is applied.RSRQ can be calculated based on the measured RSSI and RSRP using DRS.The partition of two sets of REs can be determined at UE side based on apredefined rule or higher-layer signaling. Alternatively, RSSI can bemeasured based on the partial muted REs. After the measurement, thereceiver can multiply the measured RSSI with the ratio of muting. Forexample, if 50% of RE is muted, the RSSI can be time by two in thereceiver site. The accuracy may be degraded, but it is a simpler waythan partitioning RE into two sections. The ratio of muted RE can betransmitted to receiver site via higher-layer signaling.

Fourth, RE muting can be extended to RE transmission with reduced power,as depicted by box 240. To minimize the impact to legacy UEs and reduceintroduced overhead, REs that are potential locations for DRStransmission and are not used for DRS transmission in a cell can be usedfor data transmission with reduced power in the cell to achieve reducedinterference level introduced to UEs served by other cells for DRSreception. For example, if current average cell loading is 50% and itrequires the interference level introduced by 25% average cell loadingto meet the performance requirements, REs that are potential locationsfor DRS transmission and are not used for DRS transmission can be usedfor data transmission with 50% power reduction in each cell.Furthermore, for the measurement of total received power or receivedsignal strength indicator (RSSI), part of REs that are potentiallocations for DRS transmission can be reserved and no transmission powerreduction is applied on them.

Finally, enhanced synchronization can be achieved via DRS, as depictedby box 250. For small cells with the coverage of macro cells, there ishigh likelihood for small cells to achieve time and frequencysynchronization with an offset value within a small range (e.g., ±3 μsin time and ±0.1 ppm in frequency) and no additional enhancements onsynchronization for discover reference signal design. However, for smallcells without the coverage of macrocells, there may be large time andfrequency synchronization offset between small cells and enhancements onsynchronization for discovery reference signal design may be needed. Onesimple solution to enhance the robustness of time and frequencysynchronization is to utilize existing synchronization signals. Tominimize the introduced overhead and inter-cell interference, thetransmission periodicity of synchronization signals can be increased,e.g. 50 ms, 100 ms, or the same as that of discovery reference signal.

For UE power saving, the transmission of synchronization signals can bein the neighboring time slots to or the same time slot as that where thediscovery reference signal is transmitted and small cells within alocalized area can have synchronous transmission time forsynchronization signals. For inter-cell interference coordination, smallcells within a localized area can also have different transmission timefor synchronization signals. The transmission time and periodicity ofsynchronization signals can be based on either a predefined rule orhigher-layer signaling. Whether to transmit synchronization signals insmall cells can be decided by the network and signaled to UEs throughhigher-layer signaling from the serving cell. In addition, multiplecopies of the synchronization signal can be transmitted within a periodof time, such that the receiver can average the synchronization resultsto get better timing/frequency estimation. The transmission time andperiodicity of synchronization signals can be based on either apredefined rule or higher-layer signaling.

FIG. 3A illustrates a user equipment UE 301 performing synchronization,cell detection, and measurement for small cells. UE 301 is located in aheterogeneous network with macro base station MeNB 302 and a pluralitysmall base station SeNB 311, SeNB 312, and SeNB 313. In step 321, UE 301performs network entry and establishes an RRC connection with MeNB 302.In step 322, MeNB 302 allocates a set of resource elements (REs) for DRStransmission for a plurality of small cells served by the SeNBs. Forexample, MeNB 302 determines DRS related parameters including DRSduration and DRS periodicity and assigns RE muting patterns throughcoordinating among the small cells. In step 323, MeNB 302 transmits DRCrelated configuration information to UE 301. MeNB 302 also communicatesthe information to the SeNBs via Xn Interface. In step 331 to step 333,the SeNBs transmits a set of DRS to UE 301 according to the DRSparameters. In step 341, UE 301 performs synchronization with one ormore of the small cells by detecting the corresponding synchronizationsignals in the received DRS. In step 342, UE 301 performs celldetection. Synchronization and cell detection are usually performedjointly. For example, the small cells nearby are assumed to besynchronized. UE 301 can first synchronize with the small cell with thestrongest signal strength by detecting PSS sequence. Sequence detectionis performed by using SSS and sequence detection results of PSS and SSSare combined to obtain Physical Cell ID (PCI). Further synchronizationcan be performed by using the detected SSS sequence. In addition, the UEcan perform fine synchronization using CRS and/or CSI-RS with itsserving cell. For example, if different transmission points (TP) usedifferent PCI, then CRS is sufficient. If multiple transmission pointsshare the same PCI, CRS is insufficient and CSI-RS should be used forsynchronization additionally. In step 343, UE 301 performs small cellmeasurements based on the corresponding reference signals in thereceived DRS. Because DRS comprises multiple reference signals, itenables the UE to perform synchronization, cell detection, andmeasurements during the same DRS cycle.

UE measurements on DRS includes SINR measurements and RSRP/RSRQmeasurements. The following equation (1) illustrates the calculation ofSINR based on measurements and cell loading information:

$\begin{matrix}{{SINR} = \frac{S_{i}}{{\alpha \times R} - {{RU}_{i} \times S_{i}}}} & (1)\end{matrix}$

where

-   -   S_(i) is the measured received signal strength (RSRP) of the        target cell i on DRS,    -   R is the total received power or RSSI on the selected REs,    -   RU_(i) is the average cell loading of the cell i and can be        signaled from the network to the UE.    -   α is the normalization factor depending on the number of REs        used for the measurements of R and S_(i).

The main difference between RSRQ and SINR measurement is that thedenominator of SINR does not include the received signal strength of atarget cell. SINR measurement thus requires accurate estimation ofinterference from other cells and white noise. For the numerator of SINRmeasurement for the target cell, UE can utilize REs for DRS transmissionof the target cell within the set of REs which are able to be muted ortransmitted with reduced power and are determined based on a predefinedrule or higher-layer signaling to measure the reference signal receivedpower or RSRP. For the denominator of SINR measurement for the targetcell, UE can utilize the set of REs which are potential locations forDRS transmission and are restricted from muting and are determined basedon a predefined rule or higher-layer signaling to measure total receivedpower on those REs or RSSI and then subtract the product of the measuredreference signal received power or RSRP and the average cell loading ofthe targeted cell from the measured RSSI with certain normalization. Thecell loading information can be obtained from the broadcasted messagesfrom one or multiple eNBs or from obtained from UE's own estimation.

RSRP is defined as the linear average over the power contributions ofthe REs that carry reference signal within the considered measurementfrequency bandwidth. In the multi-cell system, the signals received bythe UE in the REs that carry reference signals suffer from two types ofinterference—1) Data interference from neighboring cells; 2) referencesignal interference from neighboring cells. The performance degradationof RSRP measurement is dominated by these two types of interference. Toachieve a better performance of RSRP measurement, data interference fromneighboring cells can be reduced by applying (partial) RE mutingtechnique at the transmitter (from the eNB side) and interference ofreference signal interference from neighboring cells can be suppressedat the receiver (from the UE side) using DRS interference cancellationtechniques. To further reduce the reference signal interference fromneighboring cells, UE can measure RSRP in the OFDM symbols that containreference signal type of large frequency reuse rate.

RSRQ is defined as the ratio (N×RSRP)/RSSI, where N is the number of RBsof the measurement bandwidth and RSSI comprises the linear average ofthe total receive power in the OFDM symbols containing referencesignals. According to the definition of RSSI and RSRQ, the larger thefrequency reuse rate of the reference signal is, the larger the range ofRSRQ value is. For extreme case, RSRQ will be infinity if the frequencyreuse rate is infinity and cell loading is zero. From the perspective ofUE, large range of RSRQ value would cause the difficulty inquantization. To minimize the impact to UE, RSSI is measured in the OFDMsymbols that contain reference signal type of small frequency reuserate. In this invention, both reference signal types can be combinedinto a DRS for RSRP/RSRQ measurement. Note that the RSSI is the totalreceived power over a wideband for measurements, comparing to RSRP,which is the received power of the desired signal over a desired band.

FIG. 3B illustrates simplified block diagrams of a base station eNB 351and a user equipment UE 361 in accordance with embodiments of thepresent invention. For base station 351, antenna 357 transmits andreceives radio signals. RF transceiver module 356, coupled with theantenna, receives RF signals from the antenna, converts them to basebandsignals and sends them to processor 353. RF transceiver 356 alsoconverts received baseband signals from the processor, converts them toRF signals, and sends out to antenna 357. Processor 353 processes thereceived baseband signals and invokes different functional modules toperform features in base station 351. Memory 352 stores program codeinstructions and data 359 to control the operations of the base station.Similar configuration exists in UE 361 where antenna 367 transmits andreceives RF signals. RF transceiver module 366, coupled with theantenna, receives RF signals from the antenna, converts them to basebandsignals and sends them to processor 363. The RF transceiver 366 alsoconverts received baseband signals from the processor, converts them toRF signals, and sends out to antenna 367. Processor 363 processes thereceived baseband signals and invokes different functional modules toperform features in UE 361. Memory 362 stores program code instructionsand data 369 to control the operations of the UE.

Base station 351 and UE 361 also include several functional modules tocarry out some embodiments of the present invention. The differentfunctional modules can be implemented by software, firmware, hardware,or any combination thereof. The function modules, when executed by theprocessors 353 and 363 (e.g., via executing program codes 359 and 369),for example, allow base station 351 to configure and transmit DRS to UE361, and allow UE 361 to receive DRS and performs synchronization, celldetection, and measurements accordingly. In one example, base station351 allocates a set of radio resource for DRS transmission viaallocation module 358 and assigns RE muting patterns via RE mutingmodule 355. The DRS related configuration information is thentransmitted via configuration module 354. UE 361 receives the DRSconfiguration information via configuration module 364. UE 361 performssynchronization via synchronization module 365 and performs measurementsvia measurement module 368.

FIG. 4 illustrates a first embodiment of DRS design for small celldiscovery and measurement. In the embodiment of FIG. 4, antenna-port-0CRS pilot pattern in LTE system is reused for DRS transmission and REmuting can be applied to REs that are potential locations for CRStransmission and are not used for CRS transmission. To support both RSRPand RSRQ measurements in one subframe, REs except those for CRStransmission in a subset of OFDM symbols where CRS exists are all mutedto eliminate the data interference to CRS or transmitted with reducedpower. In Error! Reference source not found., the first OFDM symbol ofeach slot are excluded from RE muting and REs except those for actualCRS transmission in the 5^(th) OFDM symbol of each slot are all muted toavoid possible collision between data RE and CRS in different cells. Inthis example, RSRP measurement can be conducted on CRS in the 5^(th)OFDM symbol of each slot and RSSI measurement can be conducted on thefirst OFDM symbol of each slot for RSRQ calculation.

FIG. 5 illustrates a second embodiment of DRS design for small celldiscovery and measurement. In the embodiment of FIG. 5, antenna-port-0CRS pilot pattern in LTE system is reused for DRS transmission and REmuting can be applied to REs that are potential locations for CRStransmission and are not used for CRS transmission. To support both RSRPand RSRQ measurements in one subframe, REs except those for CRStransmission in a subset of OFDM symbols where CRS exists are partiallymuted or transmitted with reduced power to reduce the data interferenceto CRS. In Error! Reference source not found., the first OFDM symbol ofeach slot are excluded from RE muting and REs except those for actualCRS transmission in the 5^(th) OFDM symbol of each slot are partiallymuted to reduce the average interference level suffered by CRS by 50%.In this example, RSRP measurement can be conducted on CRS in the 5^(th)OFDM symbol of each slot and RSSI measurement can be conducted on thefirst OFDM symbol of each slot for RSRQ calculation.

FIG. 6 illustrates a third embodiment of DRS design for small celldiscovery and measurement. In the embodiment of FIG. 6, antenna-port-0CRS pilot pattern in LTE system is reused for DRS transmission and REmuting can be applied to REs that are potential locations for CRStransmission and are not used for CRS transmission. In addition to REmuting, reduced transmission power can also be applied to REs that arepotential locations for CRS transmission and are not used for CRStransmission. To minimize the impact on legacy UEs, no RE muting orreduced transmission power is applied in the 1^(st) OFDM symbol of the1^(st) slot. Error! Reference source not found. In FIG. 6, the firstOFDM symbol of the 1^(st) slot are excluded from RE muting to avoid theperformance impact on legacy control region and REs except those foractual CRS transmission in the remaining 3 OFDM symbols where there arepotential CRS transmission are all muted to avoid possible collisionbetween data RE and CRS in different cells.

FIG. 7 illustrates a fourth embodiment of DRS design for small celldiscovery and measurement. In the embodiment of FIG. 7, antenna-port-0CRS pilot pattern in LTE system is reused for DRS transmission and REmuting can be applied to a subset of REs that are potential locationsfor CRS transmission and are not used for CRS transmission. In additionto RE muting, reduced transmission power can also be applied to a subsetof REs that are potential locations for CRS transmission and are notused for CRS transmission. To minimize the impact on legacy UEs, no REmuting or reduced transmission power is applied in the 1^(st) OFDMsymbol of the 1^(st) slot. In Error! Reference source not found., thefirst OFDM symbol of the 1^(st) slot are excluded from RE muting toavoid the performance impact on legacy control region and REs exceptthose for actual CRS transmission in the remaining 3 OFDM symbols wherethere are potential CRS transmission are partially muted to reduce theaverage interference level suffered by CRS by 50%.

FIG. 8 illustrates a fifth embodiment of DRS design for small celldiscovery and measurement. In the embodiment of FIG. 8, antenna-port-6PRS pilot pattern in LTE system is reused for DRS transmission and REmuting can be applied to a subset of REs that are potential locationsfor PRS transmission and are not used for PRS transmission. In additionto RE muting, reduced transmission power can also be applied to a subsetof REs that are potential locations for PRS transmission and are notused for CRS transmission. In Error! Reference source not found., REsexcept those for actual PRS transmission in the OFDM symbols where thereare potential PRS transmission are partially muted to reduce the averageinterference level suffered by PRS by 50%.

FIG. 9 illustrates a sixth embodiment of DRS design for small celldiscovery and measurement. In the embodiment of FIG. 9, antenna-port-6PRS pilot pattern in LTE system is reused for DRS transmission and REmuting can be applied to REs that are potential locations for PRStransmission and are not used for PRS transmission. To support both RSRPand RSRQ measurements in one subframe, REs except those for PRStransmission in a subset of OFDM symbols where PRS exists are all mutedto eliminate the data interference to PRS or transmitted with reducedpower. In Error! Reference source not found., the last two OFDM symbolsof each slot are excluded from RE muting and REs except those for actualPRS transmission in the remaining OFDM symbols where there are potentialPRS transmission are all muted to avoid possible collision between dataRE and PRS in different cells. In this example, RSRP measurement can beconducted on PRS in the 4^(th) OFDM symbol of the 1^(st) slot and2^(nd), 3^(rd), 4^(th) OFDM symbols of the 2^(nd) slot and RSSImeasurement can be conducted on the last two OFDM symbols of each slotfor RSRQ calculation.

FIG. 10 illustrates a seventh embodiment of DRS design for small celldiscovery and measurement. In the embodiment of FIG. 10, antenna-port-0CRS pilot pattern and antenna-port-15 CSI-RS pilot pattern in LTE systemare reused for DRS transmission and RE muting can be applied to REswhich are potential locations for CSI-RS transmission. To support RSRPand RSRQ measurements in one subframe, ZP CSI-RSs are applied on REsexcept those for CSI-RS transmission to eliminate the data interferenceto CSI-RS for RSRP measurement and OFDM symbols where CRS exists areused for RSSI measurement. In Error! Reference source not found., exceptREs for CSI-RS transmission, ZP CSI-RSs are applied in the OFDM symbolswhere CSI-RS exists to avoid possible collision between data RE andCSI-RS in different cells. In this example, RSRP measurement can beconducted on CSI-RS in the 3^(rd) and 4^(th) OFDM symbols of the 2^(nd)slot for small cell #0, 6^(th) and 7^(th) OFDM symbol of the 2^(nd) slotfor small cell #1, 6^(th) and 7^(th) OFDM symbols of the 1^(st) slot forsmall cell #2. Timing and frequency synchronization, cell detection andRSSI measurement for RSRQ calculation can be conducted on CRS in the1^(st) and 5^(th) OFDM symbols of each slot.

FIG. 11 illustrates an eighth embodiment of DRS design for small celldiscovery and measurement. In the embodiment of FIG. 11, PSS pilotpattern, antenna-port-0 CRS pilot pattern, and antenna-port-15 CSI-RSpilot pattern in LTE system are reused for DRS transmission and REmuting can be applied to REs which are potential locations for CSI-RStransmission. PSS is used for coarse timing and frequencysynchronization. To support RSRP and RSRQ measurements in one subframe,ZP CSI-RSs are applied on REs except those for CSI-RS and PSStransmission to eliminate the data interference to CSI-RS for RSRPmeasurement and OFDM symbols where CRS exists are used for RSSImeasurement. In Error! Reference source not found., 7^(th) OFDM symbolof 1^(st) slot is reserved for PSS transmission and CSI-RS is onlytransmitted in the 2^(nd) slot. Except REs for CSI-RS transmission, ZPCSI-RSs are applied in the OFDM symbols where CSI-RS exists to avoidpossible collision between data RE and CSI-RS in different cells. Inthis example, RSRP measurement can be conducted on CSI-RS in the 3^(rd),4^(th) OFDM symbols of the 2^(nd) slot for small cell #0 and small cell#2, 6^(th), 7^(th) OFDM symbols of the 2^(nd) slot for small cell #1.Fine timing and frequency synchronization, cell detection, and RSSImeasurement for RSRQ calculation can be conducted on CRS in the 1^(st)and 5^(th) OFDM symbols of each slot.

FIG. 12 illustrates a ninth embodiment of DRS design for small celldiscovery and measurement. In the embodiment of FIG. 12, PSS pilotpattern, antenna-port-0 CRS pilot pattern, and antenna-port-15 CSI-RSpilot pattern in LTE system are reused for DRS transmission and REmuting can be applied to REs which are potential locations for CSI-RStransmission. Two copies of a PSS sequence are used for bettertiming/frequency synchronization. To support RSRP and RSRQ measurementsin one subframe, ZP CSI-RSs are applied on REs except those for CSI-RSand PSS transmission to eliminate the data interference to CSI-RS forRSRP measurement and OFDM symbols where CRS exists are used for RSSImeasurement. In Error! Reference source not found., the 6^(th) and7^(th) OFDM symbols of 1^(st) slot are reserved for PSS transmission andCSI-RS is only transmitted in the 2^(nd) slot. Except REs for CSI-RStransmission, ZP CSI-RSs are applied in the OFDM symbols where CSI-RSexists to avoid possible collision between data RE and CSI-RS indifferent cells. In this example, RSRP measurement can be conducted onCSI-RS in the 3^(rd), 4^(th) OFDM symbols of the 2^(nd) slot for smallcell #0 and small cell #2, 6^(th), 7^(th) OFDM symbols of the 2^(nd)slot for small cell #1. Fine timing and frequency synchronization, celldetection, and RSSI measurement for RSRQ calculation can be conducted onCRS in the 1^(st) and 5^(th) OFDM symbols of each slot.

FIG. 13 illustrates a tenth embodiment of DRS design for small celldiscovery and measurement. In the embodiment of FIG. 13, PSS/SSS pilotpattern, antenna-port-0 CRS pilot pattern in LTE system is reused forDRS transmission and RE muting can be applied to REs that are potentiallocations for CRS transmission and are not used for CRS transmission.PSS is used for coarse timing and frequency synchronization. SSS is usedfor fine timing and frequency synchronization and cell ID detection. Tosupport both RSRP and RSRQ measurements in one subframe, REs exceptthose for CRS transmission in a subset of OFDM symbols where CRS existsare all muted to eliminate the data interference to CRS or transmittedwith reduced power. In FIG. 13, the first OFDM symbol of each slot areexcluded from RE muting and REs except those for actual CRS transmissionin the 5^(th) OFDM symbol of each slot are all muted to avoid possiblecollision between data RE and CRS in different cells. In this example,RSRP measurement can be conducted on CRS in the 5^(th) OFDM symbol ofeach slot and RSSI measurement can be conducted on the first OFDM symbolof each slot for RSRQ calculation.

FIG. 14 illustrates an eleventh embodiment of DRS design for small celldiscovery and measurement. In the embodiment of FIG. 14, PSS/SSS pilotpattern, antenna-port-0 CRS pilot pattern, and antenna-port-15 CSI-RSpilot pattern in LTE system are reused for DRS transmission and REmuting can be applied to REs which are potential locations for CSI-RStransmission. PSS is used for coarse timing and frequencysynchronization. SSS is used for fine timing and frequencysynchronization and cell ID detection. A common method for coarsetime/frequency synchronization and cell detection is to use PSS/SSSbecause the UE is able to access 6-PRB pair channel bandwidth only inthe beginning. CRS is used for time/frequency tracking (finesynchronization) after obtaining MIB from PBCH. Since CRS sequence alsodepends on PCI, CRS can be used for cell detection as well if the UE isable to access full channel bandwidth. For example, undermacrocell-assisted small cell discovery, the UE can obtain channelbandwidth information of the small cell from the macrocell. To supportRSRP and RSRQ measurements in one subframe, ZP CSI-RSs are applied onREs except those for CSI-RS and PSS transmission to eliminate the datainterference to CSI-RS for RSRP measurement and OFDM symbols where CRSexists are used for RSSI measurement. In FIG. 14, the 6^(th) and 7^(th)OFDM symbols of 1^(st) slot are reserved for PSS transmission and CSI-RSis only transmitted in the 2^(nd) slot. Except REs for CSI-RStransmission, ZP CSI-RSs are applied in the OFDM symbols where CSI-RSexists to avoid possible collision between data RE and CSI-RS indifferent cells. In this example, RSRP measurement can be conducted onCSI-RS in the 3^(rd), 4^(th) OFDM symbols of the 2^(nd) slot for smallcell #0 and small cell #2, 6^(th), 7^(th) OFDM symbols of the 2^(nd)slot for small cell #1. Fine timing and frequency synchronization, celldetection, and RSSI measurement for RSRQ calculation can be conducted onCRS in the 1^(st) and 5^(th) OFDM symbols of each slot.

From the above embodiments, it can be seen that one DRS occasionconsists of PSS/SSS/CRS+CSI-RS. The existence of CSI-RS is configurable.The duration of one DRS occasion is one to five DL subframes. Typically,PSS/SSS and CSI-RS exists in one of the multiple subframes, while SSSexists in the 1^(st) subframe. The subframe containing CSI-RS isconfigurable. CRS exists in every DL subframe in one DRS occasion.

FIG. 15 is a flow chart of a method of resource allocation andconfiguration for DRS from eNB perspective in accordance with one novelaspect. In step 1501, a base station allocates a set of resourceelements (REs) over multiple time-domain OFDM symbols in a set of timeslots or subframes for transmitting a corresponding set of discoveryreference signals (DRS) from a plurality of small cells. In step 1502,the base station transmits configuration information to a plurality ofuser equipments (UEs). The configuration information comprisesinformation on DRS duration, DRS periodicity, and RE muting patterns ofthe set of REs for the plurality of small cells. In one embodiment, theDRS comprises at least one of a cell-specific reference signal (CRS), achannel state information reference signal (CSI-RS), a positioningreference signal (PRS), and a synchronization signal (PSS/SSS).

FIG. 16 is a flow chart of a method of small cell discover andmeasurement from eNB perspective in accordance with one novel aspect. Instep 1601, a base station transmits a set of discovery reference signal(DRS) on a corresponding set of resource elements (REs) over multipletime-domain OFDM symbols in a set of time slots or subframes. In step1602, the base station obtains DRS configuration information includingDRS duration, DRS periodicity, and RE muting patterns of the set of REs.In step 1603, the base station applies RE muting for data transmissionon a first subset of the set of REs that are not used for DRStransmission. In step 1604, the base station applies full power datatransmission on a second subset of REs. The RE muting patterns may bedetermined based on cell loading, and no RE muting is applied in controlchannels or when collide with other legacy reference signals. The REmuting patterns may be received from another base station, or determinedbased on PCI. In one embodiment, the first subset of REs belongs to afirst subset of the multiple time-domain OFDM symbols, and the secondsubset of REs belongs to a second subset of the multiple time-domainOFDM symbols.

FIG. 17 is a flow chart of a method of small cell discover andmeasurement from UE perspective in accordance with one novel aspect. Instep 1701, a user equipment (UE) receives a set of discovery referencesignals (DRS) on a corresponding set of resource elements (REs) overmultiple time-domain OFDM symbols in a set of time slots. In step 1702,the UE obtains DRS configuration information. In step 1703, the UEperforms synchronization and cell detection using a subset of DRS REsbased on the configuration information. In step 1704, the UE performsmeasurements using another subset of DRS REs based on the configurationinformation. In one embodiment, the UE performs a first measurement on afirst subset of time-domain OFDM symbols to obtain a first metric, andthe UE performs a second measurement on a second subset of time domainOFDM symbols to obtain a second metric. The first metric is a ReferenceSignal Received Power (RSRP), and RE muting is applied on a subset ofREs that are not used for DRS transmission in the first subset oftime-domain OFDM symbols. The second metric is a Received SignalStrength Indicator (RSSI), and RE muting is not applied on REs in thesecond subset of time-domain OFDM symbols. In one embodiment, a DRSduration lasts one or more subframes, and the DRS is transmitted with aperiodicity that is substantially longer than one radio frame. The UEperforms synchronization, cell detection, and measurements within onesingle DRS duration.

Although the present invention has been described in connection withcertain specific embodiments for instructional purposes, the presentinvention is not limited thereto. Accordingly, various modifications,adaptations, and combinations of various features of the describedembodiments can be practiced without departing from the scope of theinvention as set forth in the claims.

What is claimed is:
 1. A method comprising: receiving, by a userequipment (UE), a set of discovery reference signals (DRS) on acorresponding set of resource elements (REs) over multiple time-domainOFDM symbols in a set of time slots; obtaining configuration informationof the DRS; performing synchronization and cell detection using a subsetof DRS REs based on the configuration information; and performingmeasurements using another subset of DRS REs based on the configurationinformation.
 2. The method of claim 1, wherein the measurements for cellassociation are performed on different subsets of the multipletime-domain OFDM symbols and thereby obtaining different measurementmetrics of the cell based on the configuration information.
 3. Themethod of claim 2, wherein the UE performs a first measurement on afirst subset of time-domain OFDM symbols to obtain a first metric, andwherein the UE performs a second measurement on a second subset of timedomain OFDM symbols to obtain a second metric.
 4. The method of claim 3,wherein the first metric is a Reference Signal Received Power (RSRP),and wherein RE muting is applied on a subset of REs that are not usedfor DRS transmission in the first subset of time-domain OFDM symbols. 5.The method of claim 4, wherein the second metric is a Received SignalStrength Indicator (RSSI), and wherein RE muting is not applied on REsin the second subset of time-domain OFDM symbols.
 6. The method of claim5, wherein the UE derives a Reference Signal Received Quality (RSRQ) fora target cell based on RSRP and RSSI of the target cell.
 7. The methodof claim 5, wherein the UE derives a signal over interference and noiseratio (SNR) for a target cell based on RSRP, RSSI and a cell loading ofthe target cell.
 8. The method of claim 1, wherein the DRS comprises acell-specific reference signal (CRS) and a channel state informationreference signal (CSI-RS), and wherein the measurements for cellassociation is performed on the CRS and/or the CSI-RS.
 9. The method ofclaim 8, wherein the DRS further comprises a synchronization signal, andwherein the synchronization and cell detection is performed on thesynchronization signal and the CRS.
 10. The method of claim 1, wherein aDRS duration lasts for one or more subframes, and wherein the DRS istransmitted with a periodicity that is substantially longer than oneradio frame.
 11. The method of claim 10, wherein the UE performssynchronization, cell detection, and measurements within one single DRSduration.
 12. A User Equipment (UE), comprising: a receiver thatreceives a set of discovery reference signals (DRS) on a correspondingset of resource elements (REs) over multiple time-domain OFDM symbols ina set of time slots; a configuration module that obtains configurationinformation of the DRS; a synchronization module that performssynchronization and cell detection using a subset of DRS REs based onthe configuration information; and a measurement module that performsmeasurements using another subset of DRS REs based on the configurationinformation.
 13. The UE of claim 12, wherein the measurements for cellassociation are performed on different subsets of the multipletime-domain OFDM symbols and thereby obtaining different measurementmetrics of the cell based on the configuration information.
 14. The UEof claim 13, wherein the UE performs a first measurement on a firstsubset of time-domain OFDM symbols to obtain a first metric, and whereinthe UE performs a second measurement on a second subset of time domainOFDM symbols to obtain a second metric.
 15. The UE of claim 14, whereinthe first metric is a Reference Signal Received Power (RSRP), andwherein RE muting is applied on a subset of REs that are not used forDRS transmission in the first subset of time-domain OFDM symbols. 16.The UE of claim 15, wherein the second metric is a Received SignalStrength Indicator (RSSI), and wherein RE muting is not applied on REsin the second subset of time-domain OFDM symbols.
 17. The method ofclaim 16, wherein the UE derives a Reference Signal Received Quality(RSRQ) for a target cell based on RSRP and RSSI of the target cell. 18.The UE of claim 16, wherein the UE derives a signal over interferenceand noise ratio (SNR) for a target cell based on RSRP, RSSI and a cellloading of the target cell.
 19. The UE of claim 12, wherein the DRScomprises a cell-specific reference signal (CRS) and a channel stateinformation reference signal (CSI-RS), and wherein the measurements forcell association is performed on the CRS and/or the CSI-RS.
 20. The UEof claim 19, wherein the DRS further comprises a synchronization signal,and wherein the synchronization and cell detection is performed on thesynchronization signal and the CRS.
 21. The UE of claim 12, wherein aDRS duration lasts for one or more subframes, and wherein the DRS istransmitted with a periodicity that is substantially longer than oneradio frame.
 22. The UE of claim 21, wherein the UE performssynchronization, cell detection, and measurements within one single DRSduration.